The invention relates to a bell made of aluminium or aluminium alloy, respectively, having a well defined pitch of tone and long reverberation period.
Bells have been used for a many hundred years for producing musical sounds, for signalling time or the happening of some event, as well as a striking punctuating instrument, articulating the melody in an orchestra. Known forms of bells serve as idiophone instruments as well.
A bell can be considered as a complicated three-dimensional plate, which performs two-dimensional vibration when struck. In course of the two-dimensional vibration two systems of nodal lines are formed, the system of longitudinal nodal lines and the system of circular nodal lines.
The system of longitudinal nodal lines consists of nodal lines which lie in vertical planes containing the longitudinal vertical axis of the bell (when it is in its freely hanging position on the hanger), these nodal lines have approximately the same shape as the generatrices of the bell mantle and divide the bell mantle into 4 to 12 zones along the periphery. Out of the longitudinal nodal lines the largest amplitude of vibration can be observed on the nodal line which receives the stroke from the striking means, e.g. from the bell-hammer.
The other system of nodal lines arising in course of vibration consists of circular nodal lines lying in planes running perpendicularly to the longitudinal axis of the bell having been separated from each other by a space along the longitudinal axis and the centers of the circular nodal lines are lying on the longitudinal axis.
At the location of the longitudinal and circular nodal lines the bell mantle practically does not perform any oscillatory motion. Vibration modules of the longitudinal and circular nodal lines may freely combine.
On the bell, vibration with the largest amplitude is performed by the lower edge, on the flange of the bell. The flange can be considered as if it were the free edge of a clamped rod or plate. Vibration and sounds originate from the flange. The bell is almost vibrating in its entirety, with the exception of the place of holding, which is on the top, in the middle.
The tolling sound is composed of two parts: First of all the "striking sound", being a single short sound with metallic clinking, with a defined pitch of tone. The striking sound is considered the sound of the bell. After a short time the striking voice stops, decay takes place very quickly. Striking voice is the residuum of an overtone, the subjective prime tone.
Prior to complete decay of the striking sound the so-called "humming sound" enters which can be characterized by the formed state of vibration of the bell mantle, being a polyphone chord and decaying slowly. The humming sound consists of several members, mostly the prime tone, the lower sound, the third fifth and the upper octave. Out of these the lower sound is tolling for the longest time, the other ones weaken relatively fast.
The two types of bellsounds have different tonalities. The striking voice has considerable metallic clinking, meaning a plurality of non-harmonic overtones, in this case pitch of sound might be uncertain.
The humming sound is softer and it is a subdued tone. In the course of gradual decay the chord becomes increasingly clear, the sounds (overtones) are fading one after the other, at last only the lower tone tolls.
The described pattern of sounds depend on the formation of the bell mantle, it is influenced neither by the force of stroke, nor by the composition of the bell material, as these only regulate but the number and the strength of the overtones.
A bell of good quality is required to have a well defined pitch of tone and a long period of reverberation. A further requirement in respect of bell production lies in that production could be performed economically and in an easy manner, either individual pieces are produced, or serial production is desirable. With bells of known design all requirements cannot be met simultaneously.
With known bell types syntonization of striking sound and humming sound represents a difficult task. The main reason lies in that the bell mantle is vibrating around two sytems of nodal lines, being practically normal to one another and the systems of the nodal lines exert a mutual influence on the environment. Composition of sounds cannot be determined in advance by calculative methods.
Pitch of sound is determined mainly by the material and diameter of the bell, it goes without saying that the weight of the bell is proportional with the diameter. It is considered as advantageous if the prescribed required pitch of tone is achieved with a bell with a possibly less diameter and weight. A given pitch of tone with the smallest diameter and the least weight can be achieved by using gold, a larger diameter and a bell weighing more, by using silver. however, use of gold and silver cannot be considered due to the high price.
Out of the materials which are well suited to attain the desired small weight and small diameter, first there is a bronze with a Cu+Sn composition, the so-called bell-bronze should be mentioned, thereafter brass with Cu+Zn composition and lastly, siliceous copper alloy with a Cu+Si+Zn composition, i.e. a siliceous bronze material. However, these are still expensive materials, and the high price results from the fact that if used for a bell, high purity if a primary requirement. So e.g. is some hundredth percents of aluminium get into the bell-bronze, reverberation period will be reduced to one third. A further disadvantageous feature of the copper alloys lies in the utmost long time of bell casting, frequently it lasts several months, whether the bell is weighing several tons, or only a kilogram. Additionally, it is possible that even after such a long time the sound of the bell does not give the required tonality.
It has been attempted to cast bells of aluminium or aluminium alloys, however, with the known bell designs both aluminium and alloys thereof proved absolutely unsuitable, to obtain a given pitch of tone even larger diameters and weights had to be produced than with the usual copper alloys.
From the point of view of long reverberation periods the bell-bronze with a Cu+Sn composition yielded the best results, with the other copper alloys satisfactory results could not be achieved. When casting bells of aluminium or aluminium alloys and having the usual formation, the bells thus founded show unacceptable short reverberation times, in addition their sound was unsatisfactory.